This invention relates to a new process for the purification of organic polyisocyanates by mixing the polyisocyanates with certain silylated compounds. The resultant mixtures are optionally treated by degasification or worked up by distillation. These purified polyisocyanates are suitable for use in the production of polyurethane resins, and in particular, polyurethane foams.
There are various types and quantities of impurities present in polyisocyanates as a result of the production process which cause variations in activity of the polyisocyanates. The effects of these variations in activity of the polyisocyanates can be manifested in the products subsequently obtained from them, thus making reproducible and economical use of polyisocyanates difficult. In particular, the known phosgenation products of aniline/formaldehyde condensates, i.e. crude polyisocyanate mixtures of the diphenylmethane series, contain numerous such impurities. According to Chem. Soc. Rev. 3 (1974), pages 209 et seq, these are mainly impurities containing chlorine, which always give rise to fluctuations in activity if the chlorine is "highly mobile", i.e. a so-called hydrolyzable chlorine. Therefore, a reduction in the range of fluctuations in activity by reducing the quantity of these impurities so that the activity can be standardized and improved is technologically and economically important.
According to British Patent 1,080,717, the amount of hydrolyzable chlorine (HC value) can be reduced by a heat treatment at temperatures of from 180.degree. to 220.degree. C. Aside from the large consumption of energy required, such high temperature processes are dangerous due to the extremely high reactivity of polyisocyanates, which are liable to undergo oligomerization with a sudden release of heat.
It was therefore an object of the present invention to provide a new process for the purification of organic polyisocyanates which overcomes the above-mentioned disadvantages.